A DI can is one of two-piece cans that do not have a seam between its body and bottom. The DI can is obtained by ironing or redrawing/ironing a drawn can that is prepared by drawing a metal sheet. The DI can is widely used as a container for beverages such as beers and soft drinks and for foods such as soups and vegetables.
The drawing herein is a method in which a metal sheet punched into a disc is fixed using a blank holder and then formed into a cup with its bottom using a tool constituted by punches and dies in a drawing apparatus called a cupping press. The ironing is a method for thinly stretching the side wall of a formed body (cup) obtained by drawing or redrawing. A DI forming means a combination between drawing and ironing or between drawing and redrawing/ironing.
When the diameter of a metal sheet punched into a disc is much larger than that of a punch in the drawing, it may be difficult to obtain a cup having a desired shape in a single drawing. In that case, the cup is generally formed into a desired shape in a two-step drawing (drawing-redrawing). In that step, a cup having a relatively large diameter is manufactured using a drawing apparatus called a cupping press. Subsequently, redrawing is performed and ironing is then performed using a can body forming apparatus called a body maker.
Metal sheets such as a tinned steel sheet or an aluminum sheet have been commonly used as a material of a metal sheet for DI cans. After such metal sheets are DI formed into a desired shape, aftertreatments such as cleaning, surface treatment, and coating are performed to obtain a product (DI can). However, a method for manufacturing a container product (DI can) by DI forming polyester film (hereinafter may be simply referred to as “film”) laminated metal sheet (laminated metal sheet) has been considered recently to omit or simplify the aftertreatments such as cleaning, surface treatment, and coating.
DI forming methods are totally different between the case where a film laminated metal sheet is DI formed and the case where an existing metal sheet is used as a material.
As described in Japanese Unexamined Patent Application Publication No. 9-271869, an emulsion coolant is commonly used in the manufacturing of DI cans that uses an existing metal sheet as a material. Since oil is dispersed in water in this emulsion coolant, a chemical agent needs to be used for cleaning the oil left on a can surface. This easily causes damage to a film, and thus the existing emulsion coolant is unsuitable for DI forming of a laminated metal sheet.
In recent years, a water-based coolant that is excellent in ease of cleaning has been developed and commonly used as shown in Japanese Unexamined Patent Application Publication Nos. 10-85872 and 10-88176. Since the water-based coolant is utilized for DI forming that uses a metal sheet as a material, its viscosity is increased with an ester of a trihydric alcohol and a fatty acid having a carbon number of 18 (JP '872) or a polyoxyalkylene (JP '176) to improve formability by reducing friction between a metal surface and a forming tool.
However, when such a water-based coolant is utilized for DI forming that uses a laminated metal sheet as a material, there are various problems in that such a water-based coolant shows insufficient DI formability, easily causes damage to a film, and provides low food safety level of DI cans. Thus, such a water-based coolant cannot be utilized for the DI forming.
Furthermore, when a water-based coolant is used, there is a problem in that rust is easily caused on the surface of a forming apparatus for DI forming.
A method for DI forming a laminated metal sheet is totally different from a method for DI forming an existing metal sheet because the surface of a metal sheet is coated with a laminate film. In other words, the surface of the laminate film is softer than that of a metal and also has lubricity. Thus, if a high viscosity coolant containing polymers that is utilized for DI forming of an existing metal sheet is used, the DI formability is decreased.
A polyester film used for a laminated metal sheet is slightly inferior in durability against a higher fatty acid having a large number of carbon atoms. Adhesion of the polyester film to a base material decreases and the film is damaged in contact with such fatty acid having a large number of carbon atoms. In addition, the food safety level of the existing coolant itself is low because the existing coolant is used on the assumption that it is completely removed in an aftertreatment such as a cleaning step after DI forming.
Accordingly, it could be helpful to provide a DI forming water-based coolant of a laminated metal sheet that achieves excellent DI formability during DI forming of the laminated metal sheet, and satisfies the following characteristics: (i) damage is not caused to a lamination film (particularly polyester film) of the laminated metal sheet; (ii) cleaning is easily performed and a DI can with high food safety level can be obtained even if a cleaning step of DI formed parts is simplified; and (iii) rust is not easily caused on the surface of a forming apparatus in spite of a water-based coolant.
It could also be helpful to provide a method for DI forming a laminated metal sheet and a method for manufacturing a laminated DI-formed body that use such a water-based coolant.